Motor vehicles are increasingly being equipped with driver assistance systems that assist and provide support to the driver in driving the vehicle. One example of such an assistance system is a so-called adaptive cruise control (ACC) system, which automatically regulates the vehicle's speed to a desired speed selected by the driver or, if a preceding vehicle is present, adapts the speed in such a manner that a suitable distance from the preceding vehicle, monitored with the aid of a distance sensor, is maintained. Other examples of driver assistance systems are collision warning devices; automatic lane keeping systems (LKS), which detect roadway markings and automatically keep the vehicle in the center of the lane by intervening in the steering system; sensor-assisted parking aids, and the like. All these assistance systems require a sensor system with which information concerning the vehicle's vicinity may be sensed, as well as evaluation units with which that information may be suitably evaluated and interpreted.
These devices are capable of detecting objects in the vehicle's vicinity, for example other vehicles and additional obstacles, and sensing data that characterize the location and, if applicable, the motion status of those objects. The sensor systems and associated evaluation units will therefore be referred to in combination as an object sensing apparatus.
Examples of sensor systems that are used in such object sensing apparatuses are radar systems and their optical counterparts (so-called lidar systems), as well as stereo camera systems. With radar systems, the distance of the object along the line of sight may be measured by evaluating the transit time of the radar echo. The relative velocity of the object along the line of sight may also be measured directly by evaluating the Doppler shift of the radar echo. With a direction-sensitive radar system, for example a multi-beam radar, it is also possible to sense directional data concerning objects, for example the azimuth angle relative to a reference axis defined by the alignment of the radar sensor. With stereo camera systems, directional data and also (by parallax evaluation) distance data may be obtained. By evaluating the raw data measured directly by these sensor systems, it is possible to calculate data that indicate the distance of the object in the direction of travel, as well as the transverse offset of the object relative to the center of the roadway or relative to the instantaneous straight-ahead orientation of the vehicle.
Since conventional sensor systems have their strengths and weaknesses as regards sensing of the requisite measured data, it is advisable to use several sensor systems that supplement one another.
In ACC systems, it is conventional to subject the measured raw data to a plausibility evaluation in order to decide, or at least to indicate probabilities, as to whether the object sensed is a relevant obstacle or an irrelevant object, for example a sign at the side of the road. In some circumstances, an implausibility in the sensed data may also indicate a defect in the sensor system.
It general, however, it is not possible with conventional object sensing apparatuses reliably to detect misalignments or other defects in the sensor systems that negatively affect the functionality of the assistance system.